Heavy Fluid and Method of Making It

ABSTRACT

Heavy fluids are made from calcium bromide and at least one hydrogen bond donor such as a low molecular weight polyol or an organic acid. The combination of a hydrogen bond donor and calcium bromide as a hydrogen bond acceptor in an appropriate molar ratio forms a higher density clear completion fluid at a low temperature not otherwise obtainable with heavy aqueous solutions of calcium bromide such as are used in oilfield wells. A method of making the fluid comprises mixing calcium bromide with the polyol(s) in the presence of water and then reducing the water content, thus forming a heavy fluid. A crystallization inhibitor such as nitrilitriacetamide or a particulate silicate is included in the formulation. When the heavy fluid “freezes,” its physical form is somewhat amorphous and pumpable rather than crystalline. The heavy fluid is useful as a drilling fluid as well as a completion fluid and for other purposes in oil recovery processes where extreme density is beneficial.

RELATED APPLICATION

This application claims the full benefit of Provisional Application62/580,813 filed Nov. 2, 2017.

TECHNICAL FIELD

Heavy fluids are made from calcium bromide and at least one lowmolecular weight polyol which acts as a hydrogen bond donor. Thecombination of a hydrogen bond donor and calcium bromide as a hydrogenbond acceptor in an appropriate molar ratio forms a higher density clearcompletion fluid at a low temperature not otherwise obtainable withheavy aqueous solutions of calcium bromide such as are used in oilfieldwells. A method of making the fluid comprises mixing calcium bromidewith the polyol(s) in the presence of water and then reducing the watercontent, thus forming a heavy fluid. When the heavy fluid “freezes,” itsphysical form is somewhat amorphous and pumpable rather thancrystalline. The heavy fluid is useful as a drilling fluid as well as acompletion fluid and for other purposes in oil recovery processes.

BACKGROUND OF THE INVENTION

It is desirable for a clear completion fluid used in oil well processingto have a high density in order to impose a high hydrostatic pressure onthe well and counteract formation pressure. Aqueous calcium bromidesolutions are commonly used because of their weight, but simplyincreasing the concentration of CaBr₂ to obtain more weight elevates thecrystallization point—the temperature at which a solid is formed,rendering the material unpumpable, thus frustrating the fluid'susefulness.

Zinc bromide can be mixed with calcium bromide to yield higher density,but zinc bromide is a marine pollutant and highly corrosive. Fluidscontaining zinc bromide are highly corrosive both to skin and to metal.There is a compelling need to replace zinc bromide both for HSE (health,safety and environmental) reasons and to mitigate downhole corrosion. Bycomparison, calcium bromide that does not contain zinc bromide has agood HSE profile and low corrosion rates. But the fluid must have theproperties needed for the work to be done.

In offshore wells, particularly in deep water, a completion fluid islikely to be exposed to temperatures in the range of 30° to 40° F. forextended periods of time at the mud line; thus a True CrystallizationTemperature (TCT) in the completion fluid higher than 30 degrees F. isnot useful in that situation. Also, pressure can increase the TCT,compounding the need for a fluid with a low built-in TCT.

In the past, well engineers have focused on the weight concentration ofcalcium bromide in water when designing a completion fluid. That is, forexample, a 54.2% by weight CaBr₂ solution in water has a TCT of −1° F.but weighs only 14.2 pounds per gallon. Increasing the concentration ofCaBr₂ to make a 14.6 ppg solution elevates the TCT to 30° F., andincreasing the weight (density) further by simply adding more CaBr₂ toobtain a 15.3 ppg solution will elevate the TCT to 68° F., which isuseless in many wells, especially deep offshore wells. Elevatedconcentrations of calcium bromide in aqueous solution are not theanswer. A desirable practical goal is a density in the range of15.0-15.5 pounds per gallon, as it could still be handled (withappropriate precautions) and its performance is greatly enhanced by theincreased density, if its TCT can be controlled.

There is a need for a clear completion fluid which is very dense, freeof zinc bromide, and also has a low crystallization temperature or isstill pumpable when crystals form in the fluid.

As further background for the description of the invention to follow, itis of interest to remember that while pure ethylene glycol will “freeze”at about 11 or 12 degrees Fahrenheit, a 60%/40% by weight mixture ofethylene glycol and water freezes at about −49° F.; the 60/40 mixtureachieves the lowest freeze point of all ratios of the two materials.Similarly, as the ratio of glycerol to water is increased from 0/100% to100%/0, the freeze point follows a curve from 0° C. to about −46° C., ata ratio of about 66% glycerol to 34% water by weight, then increases ona substantially straight line as the ratio of glycerol to water isincreased, to the undiluted glycerol freeze point of about 17.6° C.

SUMMARY OF THE INVENTION

My invention includes a fluid having two basic components: calciumbromide and a hydrogen donor such as an organic acid or a low molecularweight polyol. While I do not intend to be bound by any theories, Ibelieve that, because of the way the composition is made, a portion ofeach ingredient is combined with some of the other to form a compositionincluding a nonsymmetric ion which is not present in a simple mixture orsolution. The formation of the nonsymmetric ion takes place by hydrogenbonding—that is, the polyol acts as a hydrogen bond donor and thecalcium bromide is a hydrogen bond acceptor. In order to achieve this,water must be removed after the mixture is made. Where substantially allthe free water is removed from the mixture, any water remaining appearsto be bound to the calcium bromide. The invention includes a method ofmaking the fluid, and the use of the fluid as a drilling fluid or as aclear completion fluid in hydrocarbon recovery.

The novel fluid includes, in addition, a small amount ofnitrilotriacetamide (“NTA”), which acts as a crystallization inhibitor;it is added at a specified point in the fluid's preparation, which isexplained further below. Alternative crystallization inhibitors such asfunctionalized or other fine particle silica, may be added after thehydrogen bonding is accomplished. The new dense fluid may also be usedas a drilling fluid. The likely formation of the nonsymmetric ion may bepart of the novel method.

I am able to incorporate a high percentage of calcium bromide in myfluid by eliminating or minimizing the use of water as a solvent orcarrier (although some may be added later for other reasons) as avehicle for introducing the CaBr₂ to the composition. In one method ofmaking my heavy fluid, water may be used first to dissolve the calciumbromide so that it can be intimately mixed with the hydrogen donor. Areadily understood example, and a quite useful and practical one, is thecombination of CaBr₂ and ethylene glycol (EG), OHCH₂CH₂OH. After theCaBr₂ is dissolved in water, the ethylene glycol is mixed with thesolution in a molar ratio of CaBr₂ to EG of 1:1. A small amount(typically 0.5% to 2.0%, based on the total non-water ingredients) ofNTA is added in aqueous solution. Water is then removed in any suitablemanner, such as evaporation, leaving a fluid having a density of about16 pounds per gallon and a modified freeze point of about 40° F. Thefluid appears to include a nonsymmetric ion made by hydrogen acting as ahydrogen bond from the hydroxyl groups of the ethylene glycol.

Evaporation, as a practical matter, requires the introduction of heat,but I may generate a portion of the heat by dissolving dry calciumbromide into a mixture of water and at least one polyol. Dissolution ofcalcium bromide in water is notably exothermic. Thus, in this mode ofpreparation, a calcium bromide solution need not be made as an initialstep. As a further essential step of my process, after dry CaBr₂ isdissolved in the mixture of water and polyol, a desired amount of wateris removed to increase the density of the mixture to at least 16 poundsper gallon, preferably at least 15 pounds per gallon.

The term “deep eutectic solvent” is used in an article by Emma L. Smith,Andrew P. Abbott, and Karl S. Ryder titled Deep Eutectic Solvents (DESs)and their Applications. See FIG. 2, a “schematic representation of aeutectic point on a two component phase diagram.” The authors note that“(t)he difference in the freezing point at the eutectic composition of abinary mixture of A+B compared to that of a theoretical ideal mixture”is “related to the magnitude of the interaction between A and B.” Theschematic shows a significant (deep) trough below a line drawn betweenthe melting points of the two components. The authors also say (p.11060-11061) that deep eutectic solvents “contain large nonsymmetricions that have low lattice energy and hence low melting points. Thecharge delocalization occurring through hydrogen bonding between forexample a halide ion and the hydrogen-donor moiety is responsible forthe decrease in the melting point of the mixture relative to the meltingpoints of the individual components.”

The deep eutectic solvents are said to be analogous to ionic liquids andare grouped into four classes in the above article and elsewhere in theliterature. Previous work with deep eutectic solvents does not, however,include the development of heavy fluids useful in oilfield applications.Nor does it include the development of heavy fluids containing suchnonsymmetric ions in a significantly water-reduced or water-deprivedstate. Nor does it include my method of creating heavy fluids that arepumpable even after reaching a low crystallization temperature. While Ibelieve nonsymmetric ions are present in all of my compositions, it willbe observed that they do not necessarily meet the definition of aeutectic, in that the crystallization point of the finished compositionmay be well above the crystallization point of the hydrogen donor.

My chief objective is to obtain a heavy fluid, free of zinc, having acrystallization point well below that of previously known solutions ofthe heavy salts used to make heavy brines and other heavy fluids used inoil well processing.

My preferred polyol is ethylene glycol. Instead of, or together withethylene glycol, propylene glycol or glycerol can be used as thehydrogen donor. In the case of glycerol, the molar ratios are adjustedaccordingly. That is, since glycerol HOCH₂CHOHCH₂OH has three (hydrogendonor) hydroxyl groups, the ideal molar ratio of CaBr₂ to glycerol wouldbe 3:2. In practice, the ratio may be varied from 5:2 to 2:5. A mixtureof at least two polyols selected from ethylene glycol, propylene glycoland glycerin, each of said polyols comprising at least 10% by weight ofthe mixture, can be used also, the particular ratios dependent on thedesired viscosity, density, cost or other properties.

Particular mixtures of polyols may also be chosen to take advantage ofthe deep eutectic freezing points such mixtures exhibit. The polyolmixtures continue to act as hydrogen donors in the presence of calciumbromide; thus, my invention includes the use of mixtures of ethyleneglycol, propylene glycol (1,2 propane diol; PG), and glycerol (GLRL) incombination of any two of them or all three wherein each is present inan amount of at least 10% by weight. Examples of such ratios are listedin List A. The invention is most efficient in hydrogen bonding to formthe above-described nonsymmetric ions when care is taken to observe themolar equivalence of hydroxyl groups to bromide moieties in thecomposition.

List A: Useful Hydrogen Donor (Polyol) Mixtures (by weight) 1. 1 GLRL:9EG 2. 1 GLRL:4 EG 3. 1 PG:9 EG 4. 1 PG:4 EG 5. 1 GLRL:3 EG 6. 1 PG:3 EG

Since glycerol and propylene glycol both contain three carbon atoms, thegeneral formula OHCH₂C(OH)₂CH₂X, where X is H or OH, may be useful todescribe the hydrogen donor combinations recited in List A. Thus theranges of list A may be summarized:

1OHCH₂C(OH)₂CH₂X, where X is H or OH:3-9EG

Likewise, combinations of ethylene glycol (EG) and low molecular weightpolyethylene glycol (having 4-8 carbon atoms and 3 to 8 hydroxylgroups—PEG) in mixtures of EG containing 1% to 25% PEG may be usefulwhere a somewhat higher viscosity is desired. As with the combinationsof EG and PG or GLRL mentioned above, the bromide should be in a rangeof near equivalence to the hydroxyl moieties, e.g. is a range of 2:5 to5:2 or preferably 3:4 to 4:5.

Other useful examples of hydrogen donors in addition to the lowmolecular weight polyols mentioned above include sorbitol, urea, citricacid, tartaric acid, and choline chloride. Where an increase inviscosity is not one of the desiderata, I prefer to use such relativelylow molecular weight materials; they tend to be more common, available,easier to work with and easier to maintain the optimum molecular weightratios; also many of them are known to be acceptable in oilfieldprocessing. Where an increase in viscosity is desired as an additionalbenefit, higher molecular weight polyols and other polymeric materialshaving many hydrogen donor sites may be used, taking care to match thehydrogen donor sites to the acceptor sites on the CaBr₂ and alsorealizing that the application in mind may have an upper limit forviscosity. The practitioner should be aware that the increasingmolecular weight of longer chain polyols will increase viscosityexponentially in aqueous solution; in the present context, where it isdesired to have little or no water, the practical limit in molecularweight for candidate polyols is rather low. Nevertheless, as indicatedabove, I do not intend to rule out the use of polyols having four ormore hydroxyl groups. Sorbitol is mentioned specifically below.

In choosing hydrogen donors, one should also be aware of their physicalstate at ambient temperatures—for example, citric acid is crystalline atroom temperature; if both the CaBr₂ and the citric acid are in the formof separate aqueous solutions added together, more water must be removedthan might otherwise be the case. Even with this example, however, thereduced-temperature crystallinity phenomenon is observable.

As indicated above, my invention includes the method of making a phasecontrolled clear heavy fluid comprising mixing a heavy hydrogen acceptorcompound with a hydrogen donor compound in the presence of water andremoving water in any effective manner to create at least some largenonsymmetric ions; evaporation is effective to remove the water andcause the formation of the nonsymmetric ions. The hydrogen donorcompound is preferably a low molecular weight polyol. By a low molecularweight polyol, I mean ethylene glycol (EG), propylene glycol (PG), andglycerol (GLRL).

I also include a small amount of a crystallization inhibitor in my densefluids. They are of two types—solid and dissolved. The dissolvedinhibitor is preferably nitrilotriacetamide (NTA) in water. The smallamount, 0.5% to 2.0% NTA, is added to the mixture before water isremoved. The solid, amorphous or functionalized silica, which may be innano size (x to y % by weight of the non-water components), may be addedbefore or after the water is removed. By inhibiting crystal formation atlower temperatures, these materials greatly enhance the ability to pumpthe dense fluid when otherwise exposure to low temperatures wouldnoticeably increase the risk that pumpability would be impossible.

The invention is further described below.

DETAILED DESCRIPTION OF THE INVENTION

Several different general methods of making my low-freezing heavy fluidwill be described below. This degree of stability appears to be due tothe formation of a nonsymmetrical ion by the action of the hydrogen bonddonor on the hydrogen bond acceptor.

Procedure 1

As indicated above, CaBr₂ and ethylene glycol may be used to make my newfluid. In order to achieve a good mix with the hydrogen donor (EG),calcium bromide is first dissolved in water, then ethylene glycol ismixed with the solution in a molar ratio of CaBr₂ to EG of 1:1. Tominimize heat expenditure in the evaporation step, the calcium bromidesolution is beneficially a saturated solution. Deviation from the 1:1ratio, say within the range 2:5 to 5:2, may be effective as a practicalmatter. Water is then removed in any suitable manner, such asevaporation, to achieve a fluid having a density of 16.5 pounds pergallon. The fluid will have a crystallization point of about 40° F.,which contrasts with the crystallization point higher than ambienttemperature which one would expect for an aqueous solution of 16.5CaBr₂.

Since it is a part of my method to remove water from the mixture, Iprefer to avoid using excess water with the CaBr₂. A saturated or nearsaturated solution is preferred, but excess water will not prevent theaccomplishment of the goal, which is to obtain a clear liquid includingas little water as possible and that includes a significant quantity ofnonsymmetric ions; these ions survive the evaporation of the water ofsolvation of the CaBr₂. Moreover, they appear to survive there-introduction of water to the fluid. If, for example, the aboveattained fluid having a density of 16.0 or 16.5 ppg is considered toodense for the particular use at hand, it is notable that the addition ofa small amount of water (or other solvent) to reduce density does notseem to affect the nonsymmetric ion relationship.

Evaporation of the water may be accomplished by heating the mixture. Anyway of heating may be used. Mixing may continue during heating andevaporating.

Procedure 2

A method of preparation similar to Example 1 substitutes glycerol forethylene glycol and incorporates calcium bromide in a molar ratio of 3CaBr₂ to 2 glycerol. Evaporating the water originally in the calciumbromide brine will result in a slightly heavier material than wasobtained using ethylene glycol.

The calcium bromide used to make the CaBr₂ solution for mixing with thepolyol hydrogen bond donor may initially be either in the hydrated oranhydride form. There are several different calcium bromide hydratesmentioned in the literature. The authors of the above-mentioned paper ondeep eutectic solvents say that the solid metal halide hydrates havelower melting points than the corresponding anhydride salts. Afterremoval of the water in my method, however, the calcium bromidemolecular structure, or at least some of it, is altered by the formationof nonsymmetric ions including both bromide and hydrogen components,and, although the entire mixture may be substantially “anhydrous,” theconventional understanding of an anhydrous molecular structure is notapplicable—a nonsymmetric ion exists in a substantially water-free orreduced water system. Particularly when solidification occurs at lowertemperatures, true crystals are not formed; the material is “soft” oramorphous and actually pumpable at the solidification temperature andbelow.

Procedure 3

To enhance the pumpability of the “frozen” eutectic material, I may adda small amount of a crystallization inhibitor such asnitrilotriacetamide (0.5-2.0% based on the non-water components),amorphous silica, functionalized silica, or any other acceptablecrystallization inhibitor. To assure even distribution of thecrystallization inhibitor, it can be present at the beginning of theprocess so that it can be thoroughly mixed into the mixture before waterremoval is begun. The silica may be added at any time in the process ofpreparation of the deep eutectic fluid.

Procedure 4

An especially useful and practical approach is to (a) prepare an aqueoussolution of calcium bromide having a density of 14.2 pounds per gallon(b) add 10 percent, based on the weight of the CaBr₂ solution, propyleneglycol (ethylene glycol, glycerol, or mixtures of the three polyols maybe substituted), and then (c) remove twenty percent, based on the weightof the whole solution, water, concentrating the solution by removing 20%of it in the form of water will result in a clear fluid having a densityof 15.2 pounds per gallon, which is a highly desirable density for manyoilfield uses. Its low crystallization temperature has been unattainablein the past for a fluid of such a density. The clear, solids-free fluidcomposition may be used as a completion fluid, drilling fluid, or forgravel packing.

For use in the oilfield, it may be convenient to use an already-preparedsolution of calcium bromide such as, for example, a 14.2 pound pergallon solution prepared off site. At the site of use, this solution canbe mixed with the hydrogen bond donor, such as one or more of the lowmolecular weight polyols mentioned above. Water is then removed from themixture to make the heavier composition of the invention. Usefulexamples of combinations of polyols include mixtures of EG with 10-15%PG or GLRL.

As my heavy fluids are contemplated for use in various oilfield and gasproduction applications, a notable advantage is that wherever water ismentioned throughout this description of the invention as a medium fordissolving calcium bromide or otherwise in the preparation of the heavyfluid, dilute brine may be used as a substitute for plain water. Dilutebrine is commonly available in the oilfield and may present a problemfor disposal which can be alleviated by use in the present invention.

Procedure 5

This procedure and Procedure 6 utilize the fact that the dissolution ofcalcium bromide in water generates heat. In this Procedure 5, which maybe particularly useful in the field, a previously prepared 14.2 poundper gallon solution of calcium bromide is mixed with the polyol and thenmore calcium bromide is added in the form of a dry salt, preferably toachieve a molar ratio described above for the final composition. Thedissolution of the calcium bromide will generate some heat and therebyelevate the temperature of the mixture, but will not be enough toevaporate any water. Additional heat will be required to evaporate therequired amount of water, create the asymmetric ions described above,and achieve a deep eutectic heavy fluid.

Procedure 6

Yet another way is to mix the three components at the same time—water,polyol, and CaBr₂. This will be even more exothermic than the approachof Procedure 5, but will still require the addition of heat to achievethe desired degree of water evaporation.

It should be understood that the crystallization inhibitors NTA andsilica are beneficially used in any of the above procedures as explainedgenerally above.

In addition to use as clear completion and drilling fluids, my heavyfluids may be useful in other oilfield applications such as pipelinecleanouts, coiled tubing cleanouts, to help release stuck drill pipe,and to remove refinery deposits. The unique combination of attributes ofmy compositions—high density, free of zinc, clear, and pumpability atvery low temperatures point to their high versatility. As to density,persons skilled in the art of hydrocarbon recovery will recognize thatthe heavy fluids may be favored candidates for any application wherebarite has been used in the past. The heavy fluid compositions mayinclude at least one additive selected from viscosity enhancing agents,corrosion inhibitors, antibacterial agents, viscosity adjusters, andhydrate inhibitors.

1. Method of making a clear, zinc-free heavy fluid comprising (a) mixing(i) calcium bromide and 0% to 50% water by weight of the total ofcalcium bromide and water with (ii) at least one hydrogen donor in amixture with up to 50% water by weight of the total of said hydrogendonor and water, (b) adding a small amount of crystallization inhibitorto the mixture of (a)(i) and (a)(ii), and (c) removing water from saidmixture to achieve a density of said mixture of at least 16 pounds pergallon.
 2. Method of claim 1 wherein said at least one hydrogen donorcomprises at least one polyol having from 2-6 carbon atoms and 2-6hydroxyl groups or at least one organic acid.
 3. Method of claim 1wherein said calcium bromide in part (a)(i) is a saturated solution ofcalcium bromide in said water.
 4. Method of claim 1 wherein said calciumbromide in part (a)(i) is solid calcium bromide.
 5. Method of claim 1wherein (1) step (a)(i) produces an aqueous solution of calcium bromidehaving a density of 13.9 to 14.5 pounds per gallon, (2) the mixture ofpart (a)(ii) comprises water and 9-11 percent of a polyol selected fromethylene glycol, propylene glycol, glycerin and mixtures thereof basedon the weight of the CaBr₂ solution, and (3) wherein, in step (c), 18-22percent water, based on the weight of the whole solution, is removed. 6.Method of claim 5 wherein the calcium bromide solution in produced instep (a)(i) has a density of 14.1 to 14.3 pounds per gallon.
 7. Methodof claim 5 wherein the mixture of part (a)(ii) comprises 9.8 to 10.2percent ethylene glycol.
 8. Method of claim 5 wherein, in step (c), 19.5to 20.5 percent of the solution is removed as water.
 9. Method of claim5 wherein, in step (c), said water is removed by evaporation.
 10. Methodof making a phase controlled heavy fluid comprising (a) mixing at leastone low molecular weight polyol with calcium bromide in the presence ofwater or dilute brine, and (b) reducing the water concentration in themixture of step (a) to obtain a fluid having (i) a crystallization pointlower than a fluid of the same proportions of the same ingredients madeby mixing without removing water, and (ii) a density of at least 15pounds per gallon.
 11. Method of claim 10 wherein said crystallizationpoint is lower than 40° F.
 12. Method of claim 10 wherein said lowmolecular weight polyol comprises ethylene glycol.
 13. Method of claim10 wherein, step (a) comprises mixing said low molecular weight polyolwith an aqueous solution of calcium bromide.
 14. Method of claim 10wherein step (a) comprises mixing in at least some of said calciumbromide as a dry salt.
 15. A clear fluid having a density of at least 15pounds per gallon, said fluid comprising ethylene glycol, calciumbromide, a small amount of crystallization inhibitor, and water, saidfluid having a crystallization temperature lower than a fluid of thesame composition and proportions made without reducing its watercontent.
 16. The clear fluid of claim 15 wherein said crystallizationinhibitor comprises nitrilotriacetamide.
 17. The clear fluid of claim 15including at least one additive selected from viscosity enhancingagents, corrosion inhibitors, antibacterial agents, viscosity adjusters,and hydrate inhibitors.
 18. The clear fluid of claim 15 including atleast one low molecular weight polyol of the formula OHCH₂C(OH)₂CH₂X,where X is H or OH in a ratio of 1 said low molecular weight polyol:3-9ethylene glycol.
 19. Method of drilling a wellbore with a drill bitcomprising circulating a fluid of claim 15 around said drill bit toremove drill cuttings.
 20. Method of completing a well comprisingcirculating a clear fluid of claim 15 through said well.